1. Field of the Invention
The present invention relates to a content reproduction device that receives and reproduces a content comprised of picture or images, sound, and the like and, more particularly, to a method and device for preventing unauthorized reproduction of a content as well as to an electronic equipment system for the same.
2. Description of the Related Art
I. Description of Terms
First of all, a brief mention is made of several terms used in a description of the technologies related to the present invention.                Real time delivery is a delivery method in which a receiving side sequentially reproduces a content (e.g., images and sound) upon receipt thereof without waiting for the transfer, from the sending side to the receiving side, of the whole content from beginning to end to be completed. If a content transfer period (time taken to transfer a content) and a buffering period to absorb delays in the transfer of the content are excluded, the time at which the content is broadcast or communicated substantially coincides with the time at which the content is reproduced. For example, the delivery of a content by streaming (hereinafter, referred to as streaming delivery) is real time delivery.        Non-real time delivery is a delivery method in which a sending side transfers the whole of a content to a receiving side first, and thereafter the receiving side reproduces the content. The reproduction start time of the content is at least a content transfer period after the transfer start time of the content. For example, delivery by downloading (hereinafter, referred to as downloading delivery) is non-real time delivery.        Reproduction rate is the amount of information (the number of bits) per unit time required to reproduce moving picture or images. For example, in a case where moving picture data is compressed for transmission as in the case of Moving Picture Experts Group (MPEG), which is one of the schemes for compressing moving-picture data, the bit rate in the compressed state (e.g., MPEG state) is the reproduction rate. The reproduction rate is an attribute of an encoded content. Hereinafter, moving-picture data is called a picture signal as appropriate.        Transmission rate is the bit rate of a signal flowing over a transmission line when the signal is transmitted through the transmission line. For example, under the service of 8-Mbps (Mega bit per second) Asymmetric Digital Subscriber Line (ADSL), the transmission rate is 8 Mbps. The transmission rate is physically fixed, depending on the transmission service in use.        Transfer rate is a communication rate between a port for an operating system (OS) program of a system delivering a content and a port for an OS program of a system receiving the content. The transfer rate is also referred to as throughput in some cases. Generally, since a network band in an intermediate network is statistically shared among communications between different pairs of host computers, the transfer rate is a value that can be obtained by averaging. The transfer rate depends on the state of congestion of a network and on the throughput of a server. Moreover, since packets are transferred through the transmission line, the sum of the transfer rates of the communications through the same transmission line is not higher than the transmission rate. Note that the transfer rate equals the transmission rate when the network band is not shared among communications between different pairs of host computers but can be exclusively used.        Program is a content for which at least a program start time (the time at which televising of a content is started), a program end time (the time at which televising of a content is ended), and a content supply method (a method for supplying a content) are determined. Note that even in a case where the program end time is not explicitly determined, it is sufficient that the program end time can be found by calculation or the like. For example, if a content reproduction period (time taken to reproduce a content) is determined, it can be calculated that the program end time is the time at which the content production period has passed since the program start time. As to the content supply method, conceivable examples include a content for which a frequency and a modulation and demodulation scheme are determined, a content for which a location and a downloading method are determined, and the like. As an example of a content for which a location is determined, a content for which a Uniform Resource Locater (URL), where the content is located, is determined can be said to be among the examples of a content for which a content supply method is determined. Incidentally, the URL explicitly indicates the location of a content, the name of a file, and a protocol to use, like “protocol name+host name+path name” to access or “protocol name+IP address+path name” to access, serving as a link to a content.        Channel is a set of a single program or a plurality of programs. Examples of the set include a set of programs the contents of which are delivered by the same entity, and a set of programs that belong to the same category although the program contents are delivered by different entities. In the case of television broadcasting, a channel is assigned to the radio frequency of a carrier wave.II. Related Art        
Studies have been made on technologies for delivering, by using a network, contents similar to those broadcast on television. For example, Japanese Patent Application Unexamined Publication No. 2005-210347 discloses, as a method for real time delivery of a moving picture signal, a streaming broadcast system that delivers moving picture and sound by streaming using packets such as Internet Protocol (IP) packets. According to this system, streaming delivery is performed after a communication band in a network is assigned, whereby loss of packets or degradation in moving picture quality due to a long delay or the like is avoided, and stably delivered data is accomplished.
Japanese Patent Application Unexamined Publication No. 2007-104588 discloses a method for delivering picture and sound by digital broadcasting using radio waves. Generally, in digital broadcasting, a frequency band is assigned to each channel. For example, in the terrestrial digital broadcasting described in this document, a communication bandwidth per channel is 6 MHz. The bit rate of information that can be transmitted within the bandwidth is determined depending on the communication bandwidth allotted, propagation state of radio waves, modulation scheme, and the like. For example, in the case of terrestrial digital broadcasting in which a bandwidth of 6 MHz is allotted, it is possible to transmit information at a maximum rate of approximately 20 Mbps when Quadrature Amplitude Modulation (QAM) and Orthogonal Frequency Division Multiplex (OFDM) are used, and if the carrier-to-noise ratio (CN ratio: the ratio of carrier frequency power to noise power) is good. Not only in this example, the fact that the bandwidth per channel is fixed imposes a limit on the bit rate at which transmission can be performed using the bandwidth in question. Accordingly, it is impossible to achieve infinite information transmission rate (bit rate).
Japanese Patent Application Unexamined Publication No. 2005-191950 discloses an example in which broadcasting over radio waves is used as a method for real time delivery and a downloading function is utilized as a method for other delivery. For example, the capability/incapability of downloading is displayed on-screen in an Electronic Program Guide (EPG), and an audience has a look at this guide and makes a click to start downloading if necessary.
Japanese Patent Application Unexamined Publication No. 2007-274318 discloses a technology in which determination is made as to whether or not to permit the reproduction of a selectively downloaded program, with the consciousness of the broadcast start time of the program. More specifically, a sending side, as a controlling entity, performs control of delivering a permission for the reproduction of a program only at or after the reproduction start time of the program.
Japanese Patent Application Unexamined Publication No. 2003-299055 discloses a video delivery system for avoiding degradation in the quality of a content due to an insufficient communication band. Specifically, a video delivery server preliminarily delivers to a subscriber accommodation device a content that requires no “real timeliness” and is scheduled to be delivered on the following day. Upon receipt of a request for the content from a subscriber terminal, the video delivery server instructs the subscriber accommodation device to send the preliminarily delivered content to the subscriber terminal.
Moreover, a network delivery video reproduction device disclosed in Japanese Patent Application Unexamined Publication No. 2008-113231 makes a request to a content delivery server for delivery of a content and receives the content encrypted. Then, when the view-enable date and time (the date and time at which the content can be viewed) has come, the network delivery image reproduction device acquires a decryption key by making a request to the content delivery server and decrypts the content by using the decryption key, thus reproducing the content.
First, in a case of displaying video upon receipt of a streaming delivery as in Japanese Patent Application Unexamined Publication No. 2005-210347 and in a case of receiving a digital television broadcast as in Japanese Patent Application Unexamined Publication No. 2007-104588, since a band to be used for communication or broadcasting is predetermined, it is impossible to deliver a content having a reproduction rate greater than the predetermined bandwidth. For example, in a case where an ADSL transmission line capable of communication at a maximum bit rate of 8 Mbps is used, it is impossible to reproduce a picture signal having a reproduction rate of 20 Mbps by real time delivery because 20-Mbps transfer cannot be made by streaming. Moreover, in a case of the current terrestrial digital broadcasting, broadcasting can be performed only at a maximum rate of approximately 20 Mbps per channel even if all of the 13 segments are used. Therefore, it is impossible to transmit/receive a broadcast of a picture signal having a reproduction rate of 30 Mbps. That is, in this case, it is impossible to receive real time delivery of a picture signal having a reproduction rate of 30 Mbps.
Moreover, in Japanese Patent Application Unexamined Publication No. 2005-191950, downloading is started in such a manner that an audience has a look at a display of the capability/incapability of downloading and then makes a click. Accordingly, when a program that the audience wants to view has not been downloaded, the downloading of the program is started then, with the result that the program cannot be reproduced immediately, or that the reproduction quality of the content is degraded. This will be described below more specifically.
For example, a case will be considered where just after a audience clicks a program content A having sufficient lead time (for example, one week) prior to reproduction but having a large file size (the transfer rate is assumed to be a), the audience comes to want to view a program content B whose broadcast start time is the point of time of clicking the program content A and whose reproduction rate is equal to the transmission rate, z. Since the transmission band is already used for the earlier started downloading of the program content A, the transfer rate that can be used for new downloading is (z-a), a value smaller than the transmission rate z by the transfer rate a of the program content A. The new usable transfer rate (z-a) is not higher the reproduction rate z (=transmission rate) of the later desired program content B. Accordingly, information cannot be transferred in the amount per unit time required to reproduce the program content B. In such a condition, even if the reproduction is performed while the file is being downloaded, the reproduction cannot be performed at the predetermined reproduction rate, resulting in degradation in picture quality (such as a frame loss and degraded resolution). Moreover, even if the program content is viewed after the downloading of the program content is completed, it is not until the downloading period has passed since the program start time of the program content that the program content can be reproduced. That is, the program content cannot be reproduced at the program start time preset by a contents provider. Accordingly, for example, when channels are changed, a content may not be able to be reproduced according to a programming schedule, depending on the status of downloading.
In a case of a program content C having a production rate higher than the transmission rate as well, since the downloading of the program content is started when a click is made at the program start time, the program content cannot be reproduced at the program start time. Alternatively, its picture is reproduced after a wait of the downloading period.
In Japanese Patent Application Unexamined Publication No. 2007-274318, a term “broadcasting device” indicates a content delivery server for downloading, and no description is given of a technology including real time delivery. Therefore, a live program cannot be delivered or broadcast. Moreover, the technology disclosed in this document is designed for selectively downloading contents. When performing selective downloading, a broadcasting device uploads a content only when a content reproduction device requests a download. Therefore, in a case where the delivery of a program content having a reproduction rate higher than the transmission rate is not completed before the program start time of this program content, when an audience changes channels to view this program after the program start time, the consequence is that the degraded picture is reproduced, or that the picture is reproduced after a wait of the downloading period, similarly to the problem with Japanese Patent Application Unexamined Publication No. 2005-191950.
As described above, according to the aforementioned technologies, it is impossible to reproduce a program content having a reproduction rate higher than the transmission rate at the program start time. In addition, a similar problem may arise with a program content having a reproduction rate higher than the transfer rate that can be used to transfer the program content. This is because, since the transfer rate is always not higher than the transmission rate, the transfer rate is not higher than the reproduction rate, and therefore, it is impossible to transmit information in the amount required for reproduction.
According to the aforementioned technologies described in Japanese Patent Application Unexamined Publication Nos. 2005-210347, 2007-104588, 2005-191950, and 2007-274318, a program content having a reproduction rate higher than the transmission rate or transfer rate cannot be reproduced at its program start time.
According to the picture delivery system disclosed in Japanese Patent Application Unexamined Publication No. 2003-299055, a picture delivery server preliminarily delivers to a subscriber accommodation device a content scheduled to be delivered on the following day and, upon request from a subscriber terminal, instructs the subscriber accommodation device to send the preliminarily delivered content to the subscriber terminal. That is, at “the time to perform preliminary delivery,” the server side checks whether or not there is any one to deliver among next day's contents and, if there is a content to preliminarily deliver, transmits the content to a subscriber accommodation device. It is the picture delivery server that determines whether or not to make a download.
In other words, according to the system in this document, the content delivery server side controls the preliminary delivery of a content. Accordingly, if a plurality of content delivery servers independently exist, and if their respective times to perform content delivery coincide, then contents having the total size exceeding a subscriber's transmission rate are to be received, sometimes resulting in a subscriber accommodation device being unable to receive the contents. Moreover, preliminarily downloaded is always a next day's content. Therefore, if a content to be downloaded for the following day has a very large amount of information, the problem may arise that the amount to be downloaded is so large that the downloading cannot be completed before the program start time.
The network delivery image reproduction device disclosed in Japanese Patent Application Unexamined Publication No. 2008-113231, as described above, acquires a decryption key for decrypting an encrypted content at the view-enable date and time and decrypts the content by using this decryption key to reproduce the content. Accordingly, the content cannot be legitimately viewed before the view-enable date and time. However, according to this document, since a content is delivered in response to a request for delivery of the content made by a user, there are some occasions when a content cannot be reproduced on time at the view-enable date and time, when a user suddenly wants to view the content that the user has not desired to.